The present invention relates generally to charged particle beam systems and more particularly to improved electrostatic deflection plates used to control of the position of the beam in such systems.
Charged particle beam systems such as cathode ray tubes, ion beam systems, electron beam systems, ink-jet printers, and the like employ magnetic and electrostatic deflection techniques to control the position of the beam. In many applications a combination of both techniques is used, for example, in the electron beam lithography system described in U.S. Pat. No. 4,494,004 to Mauer et al., entitled "Electron Beam System" and assigned to the assignee of the present invention. In this system magnetic deflection is used to move the beam from subfield to subfield through a relatively large deflection angle, while electrostatic deflection controls the movement of the beam within a subfield. The dual deflection system substantially decreases the pattern writing time without sacrificing accuracy and resolution.
The combination of electrostatic and magnetic deflection limits the choice of electrostatic deflection structures that may be employed. A solid metal plate can not be used in the presence of dynamic magnetic fields because eddy currents would be generated in the metal, which would adversely affect the deflection accuracy of the system. Induced eddy currents also present a problem in systems employing only electrostatic deflection when the switching speed of the deflection voltage is greater than about 10 MHz.
One method of reducing eddy currents is to construct the deflection plates from an insulating material which has a conductive coating, for example, plastic coated with a thin metal layer. It is important that the coating be continuous, flat, and free of defects in order to prevent unwanted deflection of the beam due to charging of exposed insulators by stray electrons. Known structures are difficult to fabricate and the coatings have exhibited blistering, cracks and chipping because of the difficulty in making the metal layer adhere to the insulating substrate. These defects, which are often microscopic in size and hard to detect, cause localized charge build-up and can result in early and frequent system failure. The thin conductive coatings are fragile and easily damaged during handling and cleaning, necessitating replacement of the deflection plates.